1. Field of Invention
The present invention relates to a current mirror technique. More particularly, the present invention relates to an apparatus for error compensation of a self calibrating current source.
2. Description of Related Art
For techniques of current source and current mirror, analog circuit, organic light-emitting diode (OLED) flat panel display and analog-to-digital converter etc. all need to use current source. FIG. 1 is a circuit diagram of a conventional self calibrating current source. N self calibrating current sources are shown in FIG. 1 and represented as CM-1˜CM-n respectively. Each of the self calibrating current sources CM-1˜CM-n includes three switches SW101˜SW103, a transistor M100 and a capacitor C100. Further, the circuit in FIG. 1 includes a current source I100.
The operation of the circuit can be classified into two stages which are referred to as a current sampling stage and a current replicating stage. First, the CM-1 circuit enters the sampling stage. During the sampling stage, switches SW101 and SW102 are turned on, and the switch SW103 is turned off. At this time, the current source I100 generates a constant current to the transistor M100. The transistor M100 biases the current value to obtain an appropriate gate-to-source voltage (Vgs), and charges the capacitor C100. When the current sampling stage of CM-1 is finished, the current sampling stages of CM-2˜CM-n are performed sequentially. Then, the circuit enters the current replicating stage. During the current replicating stage, CM-1˜CM-n are started simultaneously. At this time, the switch SW101 and the switch SW102 are turned off, and the switch SW103 is turned on. The gate of the transistor M100 receives charges stored by the capacitor C100 as the gate-to-source voltage (Vgs) of the transistor M100, so as to provide a replicated current. Assume that the gate-to-source voltage (Vgs) of the transistor M100 at this moment is identical to the gate-to-source voltage by biasing the sample current during the current sampling stage, the replicated currents output by the CM-1˜CM-n circuits are identical to the constant current generated by the current source I100.
However, in the above operation, as the switch SW101 may cause a charge injection effect, and parasitic capacitance effect may exist between switches, a charge sharing effect may occur when the switches are turned on. The above undesired effects may result in error of the voltage stored in the capacitor C100. When the transistor M100 converts the voltage into current, the error of the voltage is squared in the current, thereby resulting in an extremely large error of the replicated current. Furthermore, the capacitor C101 may have a very small current leakage, which will cause the loss of the charges stored by the capacitor and the attenuation of the stored signal after a long time of reaction, which thereby results in error of the replicated current.